The answer is…
It was plucked out of thin air, as the University of East Anglia’s Phil Jones inadvertently admitted in Climategate 2.0.
The UK Met Office subsequently admitted this as well to the Washington Post:
As Richard Betts of Britain’s Met Office has explained, defining “dangerous” global warming is a political judgment based on science rather than a purely scientific one — after all, science can’t tell us how much risk humans should accept, it can only tell us what risks we can reasonably expect to face as global temperatures rise.
As any professional in risk management can tell you, unprecedented risks cannot be reasonably anticipated. Only precedented risks, risks which we have seen before, evaluated, and have quantized, can be reasonably anticipated. This is the key step to risk mitigation, which requires changing the world to reduce the likelihood that circumstances that produce the actual risk are circumvented. Think concussions and helmets…
The temperature increasing capacity of atmospheric CO2 is known to diminish as concentrations increase. This diminution effect is probably the reason why there was no runaway greenhouse warming caused by CO2 in earlier eons when CO2 levels were known to be at levels of several thousands ppmv.
Both sceptics and Global Warming advocates agree on this. IPCC Published reports, (TAR3), acknowledge that the effective temperature increase caused by growing concentrations of CO2 in the atmosphere radically diminishes with increasing concentrations. This information has been in the IPCC reports. It is well disguised for any lay reader, (Chapter 6. Radiative Forcing of Climate Change: section 6.3.4 Total Well-Mixed Greenhouse Gas Forcing Estimate) .
Up to ~200 ppmv, the equivalent to about 82% of the temperature increasing effectiveness of CO2, is essential to maintain plant life and thus life on earth. The current level of ~400 ppmv is already committed and immutable. At that level it amounts to 93% of the warming effect of CO2 in the atmosphere.
Thus only ~7% of the effectiveness of CO2 as a warming greenhouse gas now remains.
Thus there can only ever be a minor temperature reduction impact of any de-carbonization policy, controlling CO2 emissions. Whatever political efforts are made to de-carbonize free world economies or to reduce man-made CO2 emissions, (and to be effective at temperature control those efforts would have to be universal and worldwide), those efforts can only now affect at most ~7% of the future warming effect of CO2.
The rapid diminution effect is an inconvenient fact for Global Warming advocates, nonetheless it is well understood within the climate science community but it is certainly not much discussed.
So more CO2 in the atmosphere cannot inevitably lead directly to much more warming. And increased CO2 levels in the atmosphere cannot give rise to any dangerous temperature increase.
Thus de-carbonization policies could never have useful impact to realistically control any rising world temperatures and the future world climate. As the future temperature effect of increasing CO2 emissions is now so minor, therefore there is no possibility of ever reaching the political target of less than +2.0°C.
“Two degrees is not a magical limit — it’s clearly a political goal,” says Hans Joachim Schellnhuber, director of the Potsdam Institute for Climate Impact Research (PIK). “The world will not come to an end right away in the event of stronger warming, nor are we definitely saved if warming is not as significant. The reality, of course, is much more complicated.”
Schellnhuber ought to know. He is the father of the two-degree target.
“Yes, I plead guilty,” he says, smiling. The idea didn’t hurt his career. In fact, it made him Germany’s most influential climatologist. Schellnhuber, a theoretical physicist, became Chancellor Angela Merkel’s chief scientific adviser — a position any researcher would envy.
http://www.spiegel.de/international/world/0,1518,686697,00.html
In this case, science can’t even reasonably predict the risks and benefits of 2C warming, nor whether human activity will influence such a change.